Field of the Invention
The invention relates to a mask, and more particularly, to a mask used for an evaporation process.
Description of Related Art
The organic light emitting device (OLED) has tremendous application potential to become the mainstream of the next generation flat panel display due to its advantages of self-luminescence, wide viewing angle, low power consumption, simple manufacturing process, low cost, low working temperature, high response speed, full-color display, and so forth. Generally, the method of fabricating the organic light emitting device lies in using evaporation process to form an organic material layer on the substrate, and a mask can be disposed between the substrate and the evaporation source during the evaporation process to fabricate a deposited layer that is patterned.
A fine metal mask (FMM) used for evaporation is constructed by etching a sheet metal to form micro opening therein, and the micro openings in the sheet metal after the etching are the evaporation openings which allows the evaporation materials to pass through and be deposited on the substrate. No gap shall exist between the metal mask and the substrate used for deposition during the evaporation process, so as to avoid the spillover of the evaporation material which creates shadow effect. Therefore, the evaporation process not only needs the fastening mechanism in the machine for fixing the metal mask but also needs a magnetic board disposed on the substrate, so that metal mask sticks closely to the substrate because of the magnetic force of attraction.
When the fastening mechanism fixes the metal mask, two ends of the metal mask are clamped, so as to support and to spread the metal mask. Under this condition, the metal mask undergoes the stress with a non-uniform distribution because of the metal mask pattern design (such as design about distribution of evaporation openings). When the metal mask undergoes the stress with the non-uniform distribution, the metal mask can have wrinkles and can not be able to flatly stick to the substrate. Therefore, the evaporation process needs decreasing the probability of generating wrinkles on the metal mask.